It is difficult to diagnose poorly differentiated rhabdomyosarcomas from other small round cell tumors and to identify subtypes of rhabdomyosarcomas that confer poor prognosis. Through SBIR Phase-I funding we developed and characterized monoclonal antibodies that are highly specific to MyoD, for distinguishing rhabdomyosarcomas from other malignancies. Our preliminary data using a monoclonal antibody to myogenin shows that there is a highly significant difference (p=0.0001) in staining between embryonal and alveolar rhabdomyosarcomas. We have also developed monoclonal antibodies specific to myf5 and myf6. We propose that spatio-temporal patterns of expression of members of the MyoD family (myf5, MyoD, myogenin and myf6) observed during development are reflected in rhabdomyosarcoma and may be responsible for the histologically and clinically distinct subtypes. for the Phase II study we will (1) test the diagnostic utility of monoclonal antibodies to MyoD on a large number of small round, spindle and anaplastic cell, pediatric and adult malignancies. Using the same tumor material we will also test (2) the diagnostic utility of monoclonal antibodies specific to myogenin, myf5 and myf6 for diagnosing rhabdomyosarcomas from other malignancies, (3) for distinguishing between the clinically distinct subtypes of rhabdomyosarcomas and (4) for predicting prognosis. PROPOSED COMMERCIAL APPLICATION: The use of monoclonal antibodies made to a family of tissue specific transcription factors to diagnose and subclassify tumors into clinically distinct prognostic groups is novel. This concept can be applies to various other malignancies. It offers the much needed and simple solution to the surgical pathologists and oncologists to promptly and accurately diagnose tumors and identify aggressive from non-aggressive tumors and will thus be of high commercial value.